Mechanism of Sex Determination Under Genetic Control

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Mechanism of Sex Determination Under Genetic Control!

Sex determination in most plants and animals is concerned with the study of factors which are responsible for making an individual male, female or a hermaphrodite. In the past, mechanisms of sex determination were explained purely on the basis of sex chromosomes, the constitution of which generally differed in male and female individual.

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In recent years, however, a distinction has been made firstly, between sex determination and sex differentiation and secondly between the roles played by the chromosome constitution and specific genes (located both on sex chro­mosomes and autosomes) in achieving sexual dimorphism.

It has been shown that sex determi­nation is a mere signal initiating male or female development patterns, and that sex differentiation involves the actual pathway of events leading to the development of not only male and female organs but also the secondary sex characters.

A. Mechanisms of Sex Determination Controlled Genetically:

The mechanisms of sex determination under genetic control are essentially similar in both plants and animals; the various mechanisms may be classified into following categories:

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1. Sex chromosomal mechanism or heterogamesis

2. Genie balance mechanism

3. Male haploidy or hoplodiploidy mechanisms

4. Single gene effects.

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Sex Chromosomal Mechanism (Heterogamesis):

In a vast majority of animals, male and female individuals ordinarily differ from each other in respect of either the number or the morphology of the homologues of one chromosome pair, this pair is referred to as sex chromosome or allosome.

On the other hand, those chromosomes whose number and morphology do not differ between males and females of a species are called autosoms. The sex chromosomes are responsible for the determination of sex whereas the autosomes have no relation with the sex and contain the genes which determine the somatic characters of the individuals.

There are two types of sex chromosomes: X and Y. The X- chromosome is found in both males and females, although one sex has only one while the other sex has two X chromosomes. In contrast, the Y-chromosome ordinarily occurs only in one of the two sexes of a species, e.g. male mice, Drospohila, humans etc., and female birds, reptiles, etc.

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The X-chromosomes have large amount of euchromatin and small amount of heterochromatin. The euchromatin has large amount of DNA materials, hence, much genetic information. The Y chromosome contains small amounts of euchromatin and large amount of heterochromaitin, thus having little genetic information.

Types of Sex Chromosomal Mechanism of Sex Determination:

In dioecious diploidic organisms following two systems of sex chromosomal determination of sex have been recognized;

(a) Heterogametic males

(b) Heterogametic females

Heterogametic Males:

In this type of sex chromosomal determination of sex, the female sex has two X-chromosomes, while the male sex has only one X chromosome. Because, male lacks a X chromosome, therefore, during gametogenesis it produces two types of gametes, 50 per cent gametes carry the X-chro­mosomes, while the rest lack in X chromosomes.

Such as sex which produces two different types of gametes in terms of sex chromosomes is called heterogametic sex. The female sex is therefore called homogametic sex. The heterogametic males may be of the following two types.

(i) XX-XO types:

In insects specially those of the orders Hemiptera (true bugs) and Orthoptera (grasshoppers and roaches) and certain plants (e.g., Vallisneria spiralis, Dioscorea sinuata etc.), the female have two X chromosomes (hence, referred to as XX) and are thus homogametic, while the male have only one X chromosome (hence referred to as XO).

The presence of an unpaired X chromosome determines the male sex. The male lacking in one X chroxnosomes produces two types of sperms: half of the sperms have one X chromosome; while the other half have none. The union of a sperm having a X chromosome with an egg produces a zygote having two X chromosomes (XX); such zygotes develop into female individuals. But when a sperm without an X chromosome fertilizes an egg, an XO zygote is obtained which develop into males. Thus, one half of the progeny from each mating are female, while the other halves are males.

(ii) XX-XY type:

In humans, mice, most other mammals, Hemiptera, Coleoptera, Diptera (e.g., Drosophila, house fly etc.) some fishes, some amphibia and in certain angiospermic plants such as Coccinia indica, Melandrium album, the female possesses two homomorphic X chromosomes in their body cells ((hence, referred to as XX) and they being homogametic, produce one kind of eggs, each with one X chromosome.

The males of these organisms possess one X chromosome and one Y chromosome (XY). The males having two heteromorphic sex chromosomes produce two kinds of sperms: half with X chromosome and half with Y chromosome.

Fertilization of an egg by a sperm having an X chromosome yields an XX zygote, which develops into a female. But zygotes produced by the union of an egg with a sperm having a Y chromosome produces an XY zygote which produce males.

Heterogametic Females:

In this type of sex chromosomal determination of sex, the male sex possesses two homo­morphic X chromosomes, therefore, is homogametic and produces single type of gametes, each carries a single X chromosome.

The female sex either consists of single X chromosome or one X chromosome and one Y chromosome. The female sex is thus heterogametic and produces two types of eggs, half with a X chromosome and half without a X chromosome (with or without a Y chromosome.)

The heterogametic females may be of following types:

(i) XO-XX system (XO female, XX male):

This system of sex determination is known in a few insect species, e.g., Fumea. In such species, females are the heterogametic (producing two kinds of eggs, half with a X chromosome and half without any X chromosome) and males are the heterogametic sex (producing single type of sperms, each of which carries a single X chromosome.

The union of a sperm with X chromosome containing egg oroduces XX zygote which develops into males. But fertilization of an egg devoid of a X chromosome with a sperm gives rise to a XO zygote which develops into females.

(ii) XY-XX system (XY female, XX male):

This system of sex determination operates in birds, reptiles, some insects, e.g., silk worm, etc. Here the females have XY chromosome constitution therefore it is the heterogametic sex as half the eggs have X, while the rest have a Y chromosome.

The male of these species have two X chromosomes (XX); as a result, male is the homogametic sex since all the sperms produced by males, have one X chromosome. Fertilization of a X containing egg with a sperm gives rise to a XX zygote, which develops into a male. An XY zygote is produced when a Y containing egg is fertilized by a sperm, such a zygote develops into a female.

II. Sex determination by Genie Balance:

In Drosophila, mice, man, etc., the presence of XX and XY chromosome is ordinarily associated with femaleness and maleness respectively. When a general statement is made about such situ­ations, it appears as if the specific (X and Y) chromosomes themselves determine the sex of zygotes.

However, studies on gene action leads us to expect that some specific genes located in these chromosomes should be involved in sex determination. A conclusion to this effect was reached by Bridges (1921) who finally proposed his well recognised genie balance theory of sex determi­nation in Drosophila.

In 1916, Bridges discovered XXY females and XO males in Drosophila while studying the inheritance of vermillion eye gene located in the X chromosome. This clearly showed that XX and XY chromosome constitutions were not essential for femaleness and maleness respectively, and that Y chromosome did not play a role in sex determination.

A little later, Bridges obtained triploid females; these females when mated with normal diploid males produced a number of aneuploid situations. By correlating the sex of an individual with its chromosome constitution, Bridges developed the genie balance theory of sex determination. This theory fully explains the sex determination mechanism in Drosophila, and most likely is applicable to birds as well.

The genic balance theory states that the sex of an individual is determined by a balance between the genes for maleness and those for femaleness present in the individual. In Drosophila, genes for maleness are present in autosomes, while those for femaleness are located in the X chromosome.

The genes for femaleness present in a single X chromosome are stronger than those for maleness present in one set of autosomes, that is, the haploid set of autosomes. The strength of genes for maleness and femaleness is so balanced that when the number of X chromosomes and that of autosomal sets in equal in an individual, it develops into female.

An individual develops into a male only when the number of its X chromosomes is exactly half of the number of its autosomal sets. In essence, the sex of an individual is determined by the ratio of the number of its X chromosomes and that of its autosomal sets, this ratio is termed as sex index and is expressed as follows:

As shown in the table 5.2, when the X/A ratio is 1.0, the individual will be female and if it is 0.50; it would be male. When this balance is disturbed, the sex of individuals deviates from no-mal male or normal female. For example, when the X/A ratio falls between 1.0 and 0.50, it would be intersex; when it is below 0.50, it would be supermale and when above 1.0, it would be super female.

Gynandromorphs:

Concepts of sex determination as developed for Drosophila are verified by the occasional occurrence of gynandromorphs which are individuals in which part of the body expressed male characters, whereas other parts express female characters.

The male phenotype in gynandromor­phs, on one extreme may extend to about one half of the body. In some flies; the male and female parts run longitudinally, while in some others they run transversely. Gynandromorphs are always mosaics for the X chromosome; the parts with male phenotype are always XO, while those with the female phenotype are XX.

It has been suggested that gynandromorphs arise from XX zygotes. During embryonic de­velopment in one or more cells one of the two chromosomes does not pass to any pole at anaphase and, as a result, is lost. Consequently, one or more daughter cells having a single X chromosome are produced; these cells divide and give rise to the male parts of gynandromorphs.

If the irregular distribution of X chromosome occurred at the first mitotic division of at zygote, the male phenotype would extend to exactly half the body of the gynandromorph. The extent of male part in a gynandromorph, therefore depends on the stage of embryonic development when the irregular distribution of X chromosome occurs; the earlier the occurrence, the large the size of male portion.

Sex Determination in Y-linked Genes:

In mammals, axolotl (an Amphibia) and some plants, e.g. Melandrium, the Y chromosome is essential for the development of maleness. In humans XO, XX, XXX and XXXX individuals develop the female phenotype. But in the presence of a single Y chromosome, these individuals, i.e. XY, XXY, XXXY and XXXXY, develop the male phenotype.

Thus a single Y chromosome is sufficient to overcome the effects of four X chromosomes and to produce a male phenotype. But normal human females and males are produced only by XX and XY chromosome constitutions, respectively.

The XO condition produces Turner’s syndrome (sterile female), while XXY lead to the development of Klinefelter’s syndrome (sterile male). The male determining capacity appears to be located in the short arm of human Y-chromosome (Ys); a deletion of Ys permits the development of normal female phenotype even in XY individuals. In rare cases, XX humans and XX mice exhibit a male phenotype.

In most mammals, the expression of an antigen, H-Y antigen, is closely correlated with testis development. Even XX males show the H-Y antigen, while XY females lack this antigen. The genes governing the presence of H-Y antigen is located in the short arm of Y chromosome. In case of mouse, the short arm of Y chromosome contains the sex reversal (s x r) region which is essential for the development of maleness.

III. Male Haploidy or Haplodiploidy Mechanism:

Male haploidy or haplodiploidy or arrhenotokous parthenogenesis is particularly common in the hymenopterous insects such as ants, bees and wasps (e.g., Braco?i hebetor). In these insects, since, fertilized eggs develop into diploid females and unfertilized ones into haploid males; so arrhenotoky is both a form of reproduction and a means of sex determination.

During spermatogenesis, all the n chromosomes of males regularly pass to a single pole at anaphase—I so that the opposite pole receives no chromosome at all. Thus ail the sperms are regularly haploid. Normal meiosis during oogenesis produces all haploid eggs.

Fertilization of eggs produce diploid zygotes which develop into diploid larvae. Ordinarily, such larvae give rise to workers, which are sterile females. But the diploid larvae fed on royal jelly (contains honey, pollen and some substances secreted by workers) develop into fertile females called queen. On the other hand, unfertilized eggs develop parthenogenically to produce haploid larvae and ul­timately fully fertile haploid males called drones.

IV. Single Gene Control of Sex:

In many animals, single autosomal genes override the effect of sex chromosomes present in the individuals. These genes are generally recessive, but in some cases may be dominant.

A classical example of such a gene is the autosomal recessive transfer (tra) gene of Drosophila. When this gene is present in homozygous state (tratra) it transforms XX zygotes to develop into males which are sterile. The gene tra has no effect either in the males or when it is in the het­erozygous state in females.

When a female heterozygous for tra (XX Tra tra) is mated with a male homozygous for tra (XY tra tra), only 25% of the progeny are females, while the remaining 75% are males. One-third of the males, however are sterile XX individuals homozygous for tra; they are transformed to maleness by the gene tra.

A similar recessive, possibly autosomal, testicular feminization gene induces XY humans to develop breasts and vagina; such individuals, however, have degenerate testes and are sterile. This gene does not affect the characteristics of female individuals.